Should i change the cables? And do i oughta make each 8" speaker its own chamber?

Sub-woofers should always be on their own enclosures.

This is because no two sub-woofers can be exactly in-phase with each other, so the differing frequencies at any one time can cancel each other out in the same enclosure.

And yes, try shorten the cables if you can.

Always have any cable as short as possible..

Try the enclosure without separate chanbers. Cut the cables down- they are all way too long. A more efficient cabinet will be much bigger than what you built, but space is an issue. If the speakers are any good, the cabinet might be good enough as an infinite baffle. You're asking for a lot and left out a lot. Here's a site you should spend some time with, to help you understand speakers and cabinets:

http://www.the12volt.com/caraudio/boxes.asp

By the way, cars are just about the worst listening environment you have as personal space. Some of the shortcomings can be overcome with power, but perfection will always be out of reach.